Speed



July 16, 1963 M. A. EGGENBERGER ETAL 3 097 489 FULL ARC-PARTIAL ARC TRANSFER MECHANISM FoR AN Y ELECTRO-HYDRAULIC TURBTNE CONTROL SYSTEM Flled Nov. 3, 1961 2 Sheets-Sheet l July 16, 1963 M A. EGGENBERGER ETAL 3,097,489

FULL ARC-PARTIAL ARC TRANSFER MECHANISM FOR AN- ELECTRO-HYDRAULIC TURBINE CONTROL SYSTEM Flled Nov. 3, 1961 2 Sheets-Sheet 2 FIG- 2 FULL Anc AT 40% LOAD VALVE OPENING IIO FIGB

TRANSFER AT 40% LOAD IOO/a VALVE 60 OPENING 40 IIO FIGA

PARTIAL ARC AT 40% LOAD Ooo/o" i j l 8O CV' I l 86 S.V. 87 vALvE so I I OPENING 40 85 4 LV.

2o I f o I l 1 l l n 1 IOO% |05 IIO SPEED INVENTORS'- MARKUS A. EGGENBERGER,

PAUL H. TROUTMAN,

THEIR ATTORNEY.

United States Patent Oftice 3,097,489 Patented July 16, 1963 3 097,489 FULL ARC-PARTAL 9 ARC TRANSFER MECHA- NISM FOR AN ELECTR-HYDRAULIC TURBINE CONTROL SYSTEM Markus A. Eggenberger and Paul H. Troutman, Schenectady, NX., assignors to General Electric Company, a corporation of New York Filed Nov. 3, 1961, Ser. No. 149,911 6 Claims. (Cl. titl-73) This invention relates to an arrangement for use with an electro-hydraulic :control system for a steam turbine having provisions to control the admission of steam under either full are or partial are operation, and more particularly an arrangement for shifting control between full arc and partial arc operation while also making the necessary adjustments to other valves such as the intercept valve in a reheat steam turbine.

The normal practice for controlling the admission of steam to a high-pressure steam turbine without excessive valve tbrottling losses is by the use of a number of control valves, each control valve admitting steam to a separate group `of steam nozzles. The nozzles are grouped in arcs arranged ahead of the first-stage turbine blading. Each control valve, therefore, controls the steam flow to a different location on the casing circumference.

During start-up of the turbine or during light loads, when only one `or two arcs are furnishing steam, severe -thermal stresses may be created by unbalanced heating of the casing. To alleviate this condition, full arc admission has been suggested, where all of the control valves are caused to remain open while the stop valve (nor- -m-ally used for emergency blocking of steam to the control valves) is provided with means to govern the speed and load on the turbine.

A mechanical-hydraulic arrangement for accomplishing full-arc or partial-arc admission and transferring therebetween is described in copending application Serial 843,585, tiled in the name of Markus A. Eggenberger on September 30, 1959, now Patent No. 3,027,137.

In the case of electro-hydraulic control systems for turbines, electrical signals have largely supplanted the mechanical linkages and springs found in hydraulic- -mechanical governing systems, while a tachometer generator r-ather than a mechanical flyball governor is often used to `measure actual speed. The actual speed signal is compared with a reference speed signal and an amplilied error signal operates the valves by means of suitable servo motors. Such an electro-hydraulic control system for a reheat turbine is disclosed in copending application, Serial No. 149,910, November 3, 1961, in the names of Markus A. Eggenberger, Paul H. Troutman, and Patrick C. Callan, and assigned to the assignee of the present application.

`sure turbine and is used to block the l'low of high energy steam to prevent the overspeeding of the unit after the valves controlling the admission of primary steam to the high pressure turbine have closed. Thus, the intercept valve is a `supplementary device in the iirst line of defense against overspeed and is arranged to close at a higher speed than the valves doing the primary governing, preferably commencing to close just as the other valves are closed. In turbines providing for either partial arc tor ull arc admission, the intercept valve must, therefore, be adjusted to operate over the proper speed range with respect to the valve doing the primary control- `ling of steam, whether it be the control valve or the stop valve. This correlation must exist also for dilferent regulations of the primary steam admission valves.

Accordingly, one object of the present invention is to provide an improved arrangement for shifting primary steam admission control, tor either partial arc or full arc admission in a steam turbine.

Another object of the invention is to provide for Shifting between full arc and partial arc Iadmission in a steam turbine with an electro-hydraulic control system, while also correlating the speed load relationship of the intercept valve with respect to the other valves.

Still another object of the invention is to elfect the transfer between partial arc and full arc operation at a predetermined adjustable rate.

The `subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of this application. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

FIG. 1 is a diagrammatic representation of the transfer mechanism as applied to a reheat steam turbine with an electro-hydraulic control system.

FIGS. 2 through 4 are the speed vs. valve ow characteristics of the turbine valves, illustrated at full arc, during transfer, and partial arc respectively.

Briey stated, the invention contemplates superimposing selected biasing potentials on the speed error signals controlling valve movement in order to arrange the valve characteristic curves at the proper relation with respect to the turbine rated speed at various levels of load. When eitecting transfer, the biasing potential on the valve not in control is reduced to place it in control of the turbine, and a biasing potential is then added to the valve `which was in control to remove it from control. The biasing potential applied to the intercept valve is selected, except at very light loads, to cause the intercept valve to :begin to close when the valve group which is selected to-control the admission of primary steam has just come to a fullyclosed position.

Referring now to FIG. 1 of the drawing, a turbine -generator shown generally `as 1 is under the control of an electro-hydraulic control system designated generally as 2. The arrangement of turbine `sections for-the turbine generator 1 could take many configurations but, as shown, illustrates a tandem reheat unit comprising a high pressure turbine 3, a reheat intermediate pressure turbine 4, and a double ow low pressure turbine S discharging to a condenser (not shown). Turbine :sections 3, 4 Iand 5 are shown driving a load such as the generator 6 supplying power to an electrical network (not shown). Primary steam is generated and superheated in coils 7 'in the boiler, and passes through a stop valve 8 and conduit 9. A number of control valves 10 are connected in parallel flow relationship, so that each control valve 10 controls the flow of la portion of the primary steam `through an inlet 11 to high pressure turbine section 3i. Each inlet 11 supplies steam to `a separate nozzle are (not shown) as will be understood by those skilled in the art. Although control valves 10 are shown diagrammatically as separate valves, it will be understood that usually the `valve disks will be situated 4in a common valve chest and actuated in sequence Iby a suitable mechanism. From high-pressure turbine 3, the steam is reheated in coils 12, and flows through reheat stop valve 13, and intercept valve -14 to the intermediate pressure turbine 4.

Most of the details of the turbine control system 2 are not material to the present invention. A suitable turbine Control system is disclosed in the aforementioned copendng application Serial No. 149,910, of Eggenberger et al. Briefly, a speed and load control network indicated by block 15 compares a reference speed input signal 16a, which is an electrical signal indicative of a desired turbine speed, with a negative feedback `signal 17a. The signal 17a is furnished by la tachometer generator 17 on the shaft and is indicative of turbine actual speed. An additional reference input signal 16h indicative of a desired percentage of rated load to be carried by the turbine generator is superimposed on the speed error signal which is a resultant of the comparison of signals 16a, 17a in the network 15. rIlhe error signal is further modified in network 15 to provide for such things as desired valve regulation characteristics and nonlinearity of valve iiow characteristics. Henceforth, when referring to speed error signals it will be understood that these signals are in their modied form with a load reference superimposed thereon. The result-ant modied speed error signals are furnished as electrical potentials through output leads 18, 19 and 20 to summing amplifiers 21a, 22a, 23a which, in turn, provide inputs for valve positioning servomechanisms 21, 22, 23 respectively. Each of the servomechanisms 21-23 is arranged to position one or more hydraulic rams connected to the valve stems in `accordance with the magnitude of the electric potential in input leads 1840. In addition, the servomechanism 22 is arranged to operate the parallel-connected control valves 1lb one after the other in a predetermined sequence as the magnitude of the error signal in lead 19 increases. The details of electrohydraulic servos 21-23 are not material to the present invention, but suitable arrangements are disclosed in the -aforementioned copending application Serial No. 149,910, of Eggenberger et al., or in U.S. Patent 2,977,768, issued to J. B. Wagner and Kenneth O. Straney on April 4, 1961, and `assigned to the assignee of the present application.

Therefore, the valve disk of stop valve 8 will be positioned by servo mechanism 21 in accordance with the error signal in lead 18, through a mechanical connection as indicated by the dotted line 24, so that an error sign-al indicating an actual speed which is too low will increase the valve opening to admit more steam so as to compensate for this drop -in speed. Similarly, the electrohydraulic servos 22, 23 position the valve disks of control valves and intercept valve 14 respectively through mechanical connectons `as indicated by the dotted lines 25, 26 respectively.

The transfer mechanism which is the subject of the present invention is illustrated generally `as 27. Transfer mechanism 27 includes `a 'reversible synchronous motor 28 supplied with a source of A.C. power 29 through a m'anually-operated switch 30, which may be positioned to drive the motor 28 `in one direction -by means of lead 31 or in the other direction through lead 32, as will be apparent from the drawing. A pair of contacts 33 are arranged to interrupt the current in leads 31 so as t0 limit the travel of the motor in one direction -when a solenoid relay 34 isenergized as shown. A similar pair of contacts 35 are arranged to interrupt the current `in leads 32 to limit the travel of motor 28 in the other direction when a solenoid relay 36 is de-energized.

The shaft of motor 28 operates a Variable speed drive shown generally as 37 and indicated symbolically by `a drive cone 38 and a driven cone 39, coupled by an idler 40 which may be varied by means of a manual control knob 41. The variable speed drive 37 `could take many other forms, and may also include speed reduction gearing (not shown). The variable lspeed drive 37 operates a pair of bevel gears 42 driving a lead screw 43. Thus, the lead screw 43 is driven `at an adjustable rate of speed by regulating the transfer rate knob 41. The representation of the mechanism shown is purely symbolic and is shown to illustrate the method of operation, and could take many other forms.

Disposed on the lead screw 43 -is a limit actuator 44, which operates a lever 45 to close the normally-open contacts 46 at the upper end of its travel. At the lower end of its travel, the limit `actuator 44 `strikes a spring biased lever 47, opening the normally-closed contacts 48 as shown by the dotted lines. A lead 49 connected to -a source of D.C. electric potential (not shown) is connected through the contacts 46, 48 to furnish the energizing current for solenoid relays 34, 36 respectively.

The electrical supply lead'49 also supplies the current for energizing indicator lights 50, 51, 52 on an indicator panel 53. Lamp Sil is connected in series with -a pair of contacts 54 operated by solenoid relay 36. Lamp 51 is connected in series with a pair of contacts 55 and a pair of contacts 56 operated by relays 36, 34 respectively. Lamp 52 is connected in series with a pair of contacts 57 operated by relay 34. Relay 36 operates an additional pair of contacts 18a which ground the speed error signal yappearing in lead 18.

When lead screw 43 turns, it also moves potentiometer taps 6i), 61, 62, which are electrically connected through leads `63, 64, 65 respectively, to the summing ampliiiers 21a, 22a and 23a of the electro-hydraulic servos 21, 22 and 23. The potentiometer taps 68, 61 and 62 serve to impose a supplementary transfer bias voltage, which increases the magnitude of the valve opening signals furnished through leads 18, 19 and 28 by the control network 15. `In other words, the transfer bias increases the speed which the turbine must attain before the valve closing signal starts to take effect. This supplementary transfer bias is derived from a lead 66 connected to a source of reference voltage (not shown). Bias lead 66 supplies potentiometers 67, 68, 69. Selected voltages are then taken from potentiometers 67-69 through leads 70, 71 and 72 by means of the regulating knobs 73, 74 in a manner which will be apparent from the drawing. The primary function of regulating knob 74, in addition to operating the taps of potentiometer 68, 69, is to change the regulation of the control valves in control network 15 as indicated by the dotted lines 75. Thus, when regulating knob 74 is turned to change the control valve regulation, it also changes the transfer bias voltage applied through leads 71, 72 for reasons which will be explained.

Leads 70, 71, 72 are connected to ground through tapped resistances 77, 78, 79 respectively. Thus, the supplementary transfer bias voltages as selected by regulating knobs 73, 74 are applied to the lower end of resistance 77 and to the upper ends of resistances 78, 79. Similarly, ground potential appears at the upper end of resistance 77 and at the lower ends of resistances 7-8, 79. lIt will be observed that resistance 77 is provided with an adjustable tap 80, so that a constant ground potential exists in the section of resistance 77 above the tap 80. Similarly, resistances 78, 79 are provided with fixed taps 81, `82 so that the lower portion of each of these resistances below the tap points also lies at a constant ground potential. Therefore, as lead screw taps 60--62 move downward to the dotted line positions 60a, 61a and 62a, lead 63 will have a constant ground bias while the transfer bias in leads 64, 65 will be gradually decreased. As taps 60-62 continue to move downward to the dotted line line positions 60b, 61b and 62b, the transfer bias in lead 63 will then increase while the transfer bias in leads 64, 65 remains at a constant ground bias.

It is particularly important to note that the decrease in bias on the intercept valve (lead 65) has less effect on the valve characteristic than has the decrease in bias on the control valve (lead 64). In other words, movement of the tap 6-2 a given distance changes the speed range of operation of intercept valve 14 less than the same movement of tap 61 changes the speed range of operation of the control valves 10". The purpose of this is to so correlate the adjustment of the intercept valve speed range with the adjustment required to shift from full arc to partial arc, so that the speed at which the intercept valve commences to close corresponds with the speed at which the stop valve has just closed.

A better understanding of the method by which the proper selection of supplementary transfer bias voltage and the relative resistance values of potentiometers 67- 69 and resistances 77-79 are selected may be had by reference to FIGS. 2-4 of the drawing.

yFIGURE 2 shows the percent of full valve flow plotted on the vertical scale with turbine speed plotted on the horizontal scale in percentage of rated speed. The characteristics of the valves, i.e., valve flow with respect to speed, are indicated as the diagonal lines. For example, the control valve is indicated by line 8S as having a 5% regulation, i.e., the control valve requires a 5% speed change to move the valve from its full-open position at 104% of rated speed to its fully-closed position at 109% of rated speed. It will be appreciated that line 85 represents the total flow of all of the parallel-connected control valves 10, and that only a portion of line 85 would apply to any individual control valve.

Similarly, the intercept valve characteristic is indicated by line 86 with a 2% regulation and the stop valve characteristic is indicated by line 87 with a 10% regulation. It will be observed that the stop valve line 87 passes through 100% of rated speed at 40% valve flow, i.e., 40% of full load. Thus, when the turbine is near 100% of rated speed, the stop valve is controlling the admission of primary steam to the turbine generator 1, whereas the intercept valve and control valves indicated by lines 86, 85 respectively are full open and will not commence to close until 104% of rated speed is reached.

Although regulations of 5% and 2% are indicated for the control valve and intercept valve respectively, these are only illustrative. For example, the control valve regulation is adjusted by means of knob 74 in a manner not material to the present invention. This change also readjusts the supplementary transfer biases applied to the control and intercept valves through potentiometers 68, 69, so that the speed at which the control and intercept valves close as shown on FIG. 2 will be unaffected.

The characteristics 85, S6 of control valve and intercept valve are displaced to the right on the speed axis by means of the supplementary transfer bias potentials. The bias potential for the intercept valve is selected with regard to the stop valve characteristic 87, so that the intercept valve begins to close at substantially the same speed as the speed at which the stop valve has just come to a full closed position. In other Words the finish closing speed on the graph for the stop valve and the commence closing speed on the graph for the intercept valve coincide at 104% of rated speed.

The bias potential applied to the control valve characteristic curve 85 is such as to place it at a higher speed or to the right of the intercept valve characteristic 86. `It will be observed that transfer bias supplied to the control valve characteristic 8S is such that the control valves begin to close at the same point as the intercept valve. This is not actually necessary and the control valve characteristic 8S could lie farther to the right along the speed axis. However, it is desirable to have the control valve characteristics commence at the same point as the intercept valve characteristic 86, so that the control valves can act as a second line of defense against overspeed if the stop valves fail to completely shut off the flow of primary steam.

Decrease of the supplementary transfer bias voltage on the control valve and intercept valve as the lead screw taps 61, 62 move downward will cause the characteristics 85, 86 to move `to the left on the speed scale of the graph.

FIG. 3 illustrates the situation when the lead screw taps have moved to positions 6th-62a in FIG. l, i.e. all lsupplementary transfer bias voltage has been removed. It is important -to note that the control valve characteristic 85 has moved to the left a greater extent than has the intercept valve characteristic 86. This may -be accomplished` by several means which will be apparent to 'those skilled in the tart, such as 'by employing resistors 7S, 79 of different resistance values per increment of travel.

It will also be observed that control valve characteristic S5 has moved to the left until it intersects the 100% of rated speed line at 40% of total valve flow. The intercept valve characteristic 86 has moved to the left a lesser distance, suc-h that the speed where the intercept valve commences to close (or commence closing speed) now coincides with speed at which the control valve has just closed (iinish closing speed) at 102% of rated speed. Thus, FIG. 3 shows that the relationship of the control valve characteristic with intercept valve characteristic 36 lis now the same as was previously the relationship of stop valve characteristic 87 with intercept valve characteristic 86 in FIG. 2.

As the lead screw taps continue to move downward to the dotted line positions 60b--62b in FIG. l, thecontrol valve characteristics and stop valve characteristic curves 85, 86 remain where they lare under constant zero transfer bias (due to the fixed taps 81, S2), whereas transfer bias volta-ges applied to the stop valve cause the charac- -teristic curve 87 to move gradually to the right, taking the stop valve out of control of the admission of primary steam. When the stop valve characteristic 87 has been removed from its controlling function, :a constant opening bias may be applied in lieu of a speed error signal by means of closing contacts 18a, in order to prevent unnecessary movements of the stop valve disk. This constant bias will cause the stop valve characteristic 87 to appear as a horizontal line at the full flow position in FIG. 4.

The operation of the transfer mechanism will now be described. FIG. l illustrates the turbine-generator 1 under full-arc admission operation. Full supplementary transfer bias voltage is being applied through potentiometer taps 61, 62 to the summing amplifiers 22a and 23a in order to provide the valve opening/speed characteristics indicated in FIG. 2. The bias applied to the control valve summing amplier 22a causes it to hold control valves iu a full open position for full-arc admission to all of the nozzle arcs simultaneously, while the stop valve 8 controls the admission of primary steam through the stop valve in accordance with the stop valve characteristic 87.

The supplementary transfer bias also places the intercept valve characteristic 86 as shown so that increase of speed beyond 104% of rated speed when the stop valve is completely cl-osed will cause the intercept valve 14 to commence closing and block the flow of reheated steam to the intermediate pressure turbine 4.

The limit `44 actuator on the lead screw is in its upper position as shown, so that both pairs of contacts 46, 48 are closed and both solenoid relays 34, 36 are energized as shown. The contacts 57 of relay 34 cause the full-arc indicating lamp 52 on the control panel to be lighted, indicating the status of the turbine, while the open contacts 33 prevent further rotation of motor 28.

At some value of load, for example 40% load in the example shown, it will be desired to transfer to partialarc admission in order to achieve greater efficiency of the turbine generator 1. Manual switch 30 is moved -to the partial-arc position and motor 28 will commence to turn lead yscrew 43 at a rate determined by the setting of the transfer rate knob 41. Thus, the rate of transfer from full-arc to partial-arc is at a predetermined rate -which may be on the order of l to 10 minutes.

As lead screw 43 turns, contacts 46 open and solenoid 34 is de-energized, closing contacts 56 and opening contacts 57, which will de-energize full arc indicator lamp 52 and energize the transfer indicator lamp 51. As the lead screw taps 60-62 travel downward to the dotted line posi-tions indicated by 60a-62a, lthe control valve characteristic 85 will move to the left on the speed scale at a more rapid rate than the intercept valve characteristie S6, until the characteristics assume the position shown in FIG. 3. Since the control valve has a narrower regulation than the stop valve in the example given, i.e., a smaller speed change is required for full valve travel, it will actually be in control at this point.

However, the stop valve 8 will still exercise some measure of control since it aects the pressure of the steam supplied through conduit 9 to control valves l0.

As the lead screw taps @Q62 continue to travel to the positions 6flb-62b, transfer bias voltage will now be applied to move the stop valve characteristic S7 to the right on the `speed curve. As the limit actuator 441 on the lead screw 43 reaches the lower end of its travel, it opens contacts 4S, de-energizing solenoid 36, to close contacts 18a. This grounds the speed error signal to the stop valve appearing in lead 18 and leaves the constant opening lbias `as the only signal to the summing amplifier 21a. This has the effect of making the stop valve characteristic in FIG. 4 appear as the horizontal line 87. De-energization of solenoid relay 36 also opens the transfer indicator lamp contacts 55 and closes contacts 54, lighting the partial-arc indicator lamp Sil, indicating that the turbine is now in this conditon.

It `only remains to note that the rate at which transfer is taking place may be adjusted at any time by moving knob 4l to speed up or slow down the transfer, Also, the transfer may be interrupted by moving manual switch 30 to the hold position. To return from partial-arc admission to full-arc admission, the manual switch 39 is moved to full are and the vsame sequence of events takes place, but in reverse order.

Thus, it will be seen that, by the use of the transfer mechanism disclosed, transition from full-arc to partialarc and back again is accomplished smoothly and at an adjustable predetermined rate. Also, the operating speed range `of the intercept valve may be correlated with whichever valve is `controlling the `admission of primary steam, so that the commence closing speed of the intercept valve is the same as the finish closing speed of the primary steam admission valve (i.e., either the stop valve or the control Valve). This is highly desirable in yorder to block the fiow of reheat steam turbine 4 to the intermediate pressure turbine 4 after the primary steam admitting valve has closed should load on .the generator be lost during the transfer.

'Dhe switch 30, although in the present disclosure shown manually operated, can be operated by a logic system which senses the conditions calling for full arc or partial arc admission and, therefore, can be used to make the entire system fully automatic.

While only one embodiment of the invention has been illustrated for purposes of explanation, it will be apparent to those skilled in the art that other modifications could be made, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of lche United States is:

1. In an electro-hydraulic turbine control system of the type having a plurality of steam valves controlling the Yflow of steam to a turbine and positioned in accordance with electrical signals representative of the error between desired turbine speed and actual turbine speed at a given load, the combination of stop valve means positioned by a first electrical signal derived yfrom a speed error indication,

a plurality of control valve means supplied with steam through said stop Valve means, and sequentially positioned by a second electrical signal derived from the same speed error indication,

and means superimposing selected first and second transfer biasing electrical signals on either the first signal or the second signal respectively in a sense to increase the speed at which said first and second signals take effect, whereby only one of said valve means controls the iiow of steam at a given turbine speed while the other of said valve means remains open.

2; In an electro-hydraulic turbine control system of the type having a plurality of steam valves controlling the flow of steam to a turbine and positioned in accordance with electrical potentials representative of the error between desired turbine speed and actual turbine speed at a given load, the combination of stop valve means positioned by a first electrical potential derived from a speed error indication,

a plurality of control valve means each supplied with steam through said stop valve means and sequentially positioned by a second electrical potential derived from the same speed error indication,

a source of supplementary first and second transfer biasing electrical potentials of a polarity to increase the magnitude of said error potentials in a valve opening direction,

and means including first and second variable impedance devices connected to said source of biasing potential for first gradually removing said first biasing potential from said first speed error dependent potential, and then gradually adding said second biasing potential to said second speed error dependent potential, whereby control over steam fiow at a given turbine speed and load may be shifted between said valve means.

3. yIn an electro-hydraulic turbine control system of the type having a plurality of steam Valves controlling the flow of steam to a turbine and positioned in accordance with electircal potentials representative of the error between desired turbine speed and actual turbine speed at a given load, the combination of stop valve means positioned by a first electrical potential `derived from a speed error indication,

control valve means including a plurality of valves each supplied through said stop valve means and sequentially positioned by a second electrical potential derived from the same speed error indication,

intercept valve means indirectly supplied by said con- -trol valve means and positioned by a third electrical potential derived from the same speed error indication,

means superimposing selected first and second transfer biasing electrical potentials on either the first speed error dependent potential or the second speed error dependent potential respectively in a sense to increase the speed at which said speed error dependent potential takes effect, whereby only one of either said control valve means or said stop valve means controls the flow of steam at a given speed and load While the other of said valve means remains open,

and means superimposing a third transfer biasing electrical potential on said third speed error dependent potential of a magnitude such that said intercept valve commences to close at substantially the same speed that the valve means controlling steam iiow to the turbine just closes.

4. =In an electro-hydraulic turbine control system of the type having a plurality of steam valves controlling the ow of steam to a turbine and positioned in accordance with electrical potentials representative of the error between desired turbine speed and actual turbine speed at a given load, the combination of:

stop valve means positioned by a first electrical potential derived from a speed error indication,

control valve means including a plurality of valves each supplied by said stop valve means and sequentially positioned by a second electrical potential derived from the same speed error indication,

intercept valve means indirectly supplied by said control valve means and positioned by a third electrical potential derived from the same speed error indication,

a source of first, second and third transfer biasing electrical potentials of a polarity to increase the magnitude of said first, second and third speed error dependent potentials in a valve opening `direction for the stop valve means, control valve means, and intercept valve means respectively,

and biasing transfer means including a plurality of variable impedance devices connected to superimpose said first, second and third biasing potentials on said first, second and third speed error `dependent potentials respectively, said transfer means including a reversible motor driving said variable impedance devices to first gradually remove the second biasing potential from said second speed error dependent signal, and then to apply said first biasing potential to said first speed error dependent signal, said transfer means also being arranged to change the value of the third biasing potential such that the intercept valve commence closing speed is moved Ifrom substantial coincidence with stop valve finish closing speed to substantial coincidence with control valve finish closing speed.

5. The combination according to claim 4 wherein said transfer means reversible motor drives said variable impedance devices through a variable speed drive, whereby the time over which said transfer means acts may be adjusted.

6. The combination of a reheat steam turbine having a high-pressure turbine section with circumferentially spaced nozzle arcs and a lower pressure reheat turbine section,

stop valve means controlling the flow yof steam to the turbine,

a plurality of control valve means each supplied in common through said stop valve means `and separately connected to said nozzle arcs in said high-pressure turbine section,

intercept valve means controlling the fiow of reheated steam from the high-pressure turbine section to said lower pressure reheat turbine section,

first, second and third electro-hydraulic servo means connected to position said stop valve means, said control valve means, and said intercept valve means respectively in response to electrical potentials.

a control network comparing a speed reference electrical potential representative of desired turbine l@ speed with a feedback electrical potential` representative of turbine actual speed to derive a speed error signal and modifying,

said speed error signal to derive first, second, and third speed error dependent electrical potentials representative of the error between desired speed and actual speed at ya given load, said first, second, and third speed error dependent potentials being supplied to said iirst, second and third electro-hydraulic servo means respectively,

a source of bias voltage providing first, second and third transfer biasing electrical potentials of a polarity to increase the magnitude of said iirSt, second and third speed ernor dependent potentials respectively in a valve opening direction,

first, second and third variable impedance devices connected for varying the magnitude of said first, second and third transfer biasing potentials applied to said first, second and third speed error dependent potentials respectively,

reversible motor means driving said variable impedance devices together at a preselected adjustable rate of speed, said tirst impedance device being arranged to provide a constant minimum biasing potential while the second and third impedance devices furnish a biasing potential varying from maximum to a minimum potential, said second and third impedance devices then providing a constant minimum biasing potential while the first impedance device furnishes a biasing potential varying from minimum to maximum, the effect of variance iof the third biasing potential being selected with respect to the effect or variance of the second biasing potential such that the intercept valve commerce closing speed is moved from substantial coincidence with the stop valve nish closing speed to substantial coincidence with the control Valve finish closing speed.

No references cited. 

1. IN AN ELECTRO-HYDRAULIC TURBINE CONTROL SYSTEM OF THE TYPE HAVING A PLURALITY OF STEAM VALVES CONTROLLING THE FLOW OF STREAM TO A TURBINE AND POSITIONED IN ACCORDANCE WITH ELECTRICAL SIGNALS REPRESENTATIVE OF THE ERROR BETWEEN DESIRED TURBINE SPEED AND ACTUAL TURBINE SPEED AT A GIVEN LOAD, THE COMBINATION OF: STOP VALVE MEANS POSITIONED BY A FIRST ELECTRICAL SIGNAL DERIVED FROM A SPEED ERROR INDICATION, A PLURALITY OF CONTROL VALVE MEANS SUPPLIED WITH STEAM THROUGH SAID STOP VALVE MEANS, ANS SEQUENTIALLY POSITIONED BY A SECOND ELECTRICAL SIGNAL DERIVED FROM THE SAME SPEED ERROR INDICATION, 